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Alkane Complexes As Intermediates in C-H Bond Activation Reactions

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Alkane Complexes As Intermediates in C-H Bond Activation Reactions ACS Symposium Series 885, Activation and Functionalization of C-H Bonds, Karen I. Goldberg and Alan S. Goldman, eds. 2004. Chapter 3 Alkane Complexes as Intermediates in C-H Bond Activation Reactions William D. Jones*, Andrew J. Vetter, Douglas D. Wick, Todd O. Northcutt Department of Chemistry, University of Rochester, Rochester, NY 14627 The rearrangements of alkyl deuteride complexes are monitored and a kinetic model used to extract the rate constants for the fundamental processes involving the unseen alkane !-complexes. Isotope effects for C-H and C-D bond formation and cleavage have been measured to permit these determinations. The relative rates of the processes available to and alkane !-complex (C-H oxidative cleavage, C-D oxidative cleavage, dissociation, migration to an adjacent C-H bond) have been determined for methane, ethane, propane, butane, pentane, and hexane. Introduction It has now been generally established that alkanes are activated by homogeneous transition metal complexes by way of initial complexation of the C-H bond to the metal, followed by oxidative cleavage of the C-H bond to produce an alkyl hydride product (eq 1) (1). Evidence for this pathway has come from a variety of observations, including scrambling of a metal-deuteride © 2004 American Chemical Society 56 57 into the "-position of the alkyl group (2-10), temperature-dependent linewidth variations (11), and direct observation via NMR or IR spectroscopy (12,13). Despite these studies, little is known about the behavior of these alkane complexes due to their lability and the inability to determine the structure (i.e. binding site) in these species. In this article we present an overview of experiments that use deuterium scrambling to monitor the rearrangements of alkyl hydride complexes. Through rate measurements and kinetic modeling, it has proven possible to extract the relative rates of the various processes open to the alkane complex intermediates. -L R-H Keq R Ln+1[M] [LnM] [LnM(R-H)] LnM (1) H The direct observation of alkane complexes has been limited to a few experiments involving low temperature spectroscopy or transient absorption techniques. One of the more interesting of these is the report by Ball in which photolysis of CpRe(CO)3 in cyclopentane at 180 K leads to the observation of a !-cyclopentane complex (12). A resonance is observed at # -2.32 for two hydrogens interacting with the metal center, suggesting either an $2-H,H or a fluxional $2-C,H structure as shown below (eq 2). Time resolved IR experiments using CpRe(CO)3 in heptane solvent at 298 K have also shown evidence for a transient intermediate assigned as the !-heptane complex. The species has an approximate lifetime of 10's of milliseconds before back reacting with CO in a bimolecular reaction (13). h! fast H Re Re H or H Re OC CO OC Re OC (2) c-C5H10 OC OC H H OC OC H OC Another interesting example where a !-alkane complex has been observed by transient IR spectroscopy is in the photolysis of Tp*Rh(CO)2 in cyclohexane solution (Tp* = tris(3,5-dimethylpyrazolyl)borate) (14,15). A species assigned as the alkane complex is observed at 1990 cm-1 that converts to the alkyl hydride complex with a half-life of 230 ns. Conversion of the $3-Tp* ligand to an $2- Tp* ligand is seen to occur on the 200 ps timescale (eq 3). As with the IR study above, however, little can be ascertained regarding the structure of the alkane complex. Fast IR studies of Cp*Rh(CO)2 and alkanes in liquid xenon and krypton solvents provide evidence for the intermediacy of alkane complexes of the type Cp*Rh(CO)(alkane), also characterized through the carbonyl IR absorptions (16-18). © 2004 American Chemical Society 58 R-H R 3 h" [ 3-Tp*Rh(CO)] 3-Tp*(CO)Rh ! -Tp*Rh(CO)2 ! ! -CO fast H 1972 cm-1 (3) 200 ps fast 230 ns R 3 R 2 R 2 ! -Tp*(CO)Rh ! -Tp*(CO)Rh ! -Tp*(CO)Rh H H H -1 -1 2032 cm 1990 cm Our research group has been studying a related complex, [Tp*Rh(CNR)], for C-H bond activation reactions over the past decade (CNR = neopentyl isocyanide). The fragment can be conveniently prepared by irradiation of the carbodiimide adduct, Tp*Rh(CNR)(RN=C=NPh), which has nearly unit quantum efficiency for loss of the carbodiimide ligand (19). Early studies with benzene activation to produce the phenyl hydride complex Tp*Rh(CNR)PhH were proposed to occur by way of an unobserved $2-benzene adduct, not unlike the mechanism proposed for alkane activation (20). Evidence for this adduct came from the observation that the phenyl deuteride complex was found to scramble the deuterium into the ortho, meta, and para positions of the phenyl ring much faster than the rate of benzene exchange. In addition, displacement of benzene from Tp*Rh(CNR)PhH by isocyanide was found to be an associative process with activation parameters consistent with attack of isocyanide on the metal complex. These experiments suggest a pre-equilibrium with the $2- benzene complex prior to further reaction (eq 4). The associative kinetics were N N + CNR N N CNR - C6H6 H B Rh N N CNR H B N N N N N N N N rapid N N Rh H B Rh CNR reversible N N H (4) H CNR B N N N N N N + C6D6 Rh - C6H6 D CNR d5 © 2004 American Chemical Society 59 most consistent with the formulation of a d8 square planar Rh(I) intermediate, and such rapid $3 – $2 interconversions are consistent with the earlier interpretations of the carbonyl derivative by Bergman and Harris (14). The rhodium carbodiimide precursor to [Tp*Rh(CNR)] was also found to be valuable for the activation of alkanes, including methane, propane, pentane, cyclopentane, and cyclohexane (21). The complexes Tp*Rh(CNR)(R)H have stabilities toward alkane reductive elimination at ambient temperature in benzene solution on the order of minutes (for cyclohexane) to hours (for methane). In the case of the linear hydrocarbons, only n-alkyl products are formed, even at low temperatures. Yet the formation of cyclopentyl and cyclohexyl hydride adducts indicated that activation of secondary C-H bonds was possible. We set out to determine and understand the stabilities of the different isomers available in the alkane oxidative addition adducts, as well as the role that the !-alkane complexes played in their formation and interconversions. The use of rhodium deuteride derivatives proved to be a valuable tool in these studies, as they are readily prepared by the metathesis reaction of the rhodium chloride complex (e.g. Tp*Rh(CNR)MeCl) with Cp2ZrD2, as reported in our full paper (22). In addition, the use of deuteride compounds to monitor the rearrangements introduced isotope effects on the rate of reaction, and these needed to be determined and taken into account in order to provide a meaningful interpretation of the observations. A full discussion of isotope effects in these reactions has recently appeared (23). Isotope Effect Measurements One of the key experiments that permitted our success in this project was the ability to prepare an isopropyl hydride complex from the corresponding isopropyl chloride complex. The metathesis reaction with Cp2ZrH2 was not immediate, but was fast enough to allow for complete formation of the isopropyl hydride complex in about 10 minutes. Furthermore, the isopropyl hydride complex was seen to convert into the n-propyl hydride complex competitively with loss of propane over the next hour or so. The unsaturated fragment reacted with the benzene solvent to give ultimately Tp*Rh(CNR)PhH as the thermodynamically most favored product (Figure 1). This experiment demonstrated that secondary alkyl hydride complexes have sufficient stability that they should have been observed during the photolysis of Tp*Rh(CNR)(RN=C=NPh) in propane solvent to generate Cp*Rh(CNR)(n- propyl)H. Taken together, these experiments prove that activation of secondary C-H bonds does not occur in linear alkanes. A second related experiment was then undertaken using Cp2ZrD2 to prepare the isopropyl deuteride complex. A similar sequence of events is seen: © 2004 American Chemical Society 60 100 75 [Cp ZrH ] kbc l 2 2 2 [Rh] [Rh] a [Rh] t C6D6 o H H t Cl kab f 50 o kbd kcd % 25 C6 D5 [Rh] D 0 0 50 100 150 200 250 time, min Figure 1. Rearrangement of isopropyl hydride complex conversion of the isopropyl deuteride complex to the n-propyl hydride complex with competitive loss of propane-d1 and formation of Tp*Rh(CNR)PhH. In this case, however, it is important to point out that no scrambling of deuterium is seen in the isopropyl deuteride complex. That is, one does not observe the formation of Tp*Rh(CNR)(CDMe2)H as Tp*Rh(CNR)(CHMe2)D goes away. It implies that the reductive coupling in the isopropyl deuteride (and hydride) complex is irreversible (eq 5). From this, we conclude that reductive coupling of a secondary alkyl hydride complex is irreversible. This observation is important, and is unique in the C-H activation literature. Furthermore, since the reaction is irreversible, the rate of disappearance of the isopropyl deuteride (or hydride) can be used to determine the fundamental rate of reductive coupling. From the ratio of these rates in the deuteride and hydride complexes, we can therefore determine the isotope effect in just the reductive coupling step, without H D complications from reversibility. From our measurements kRC /kRC = 2.1. H D [Rh] [Rh] D H [Rh] = Tp*Rh(CNR) (5) H D C6D5 [Rh] D + [Rh] D H © 2004 American Chemical Society 61 We have also been able to determine the isotope effect on the reverse reaction in this system, namely oxidative cleavage.
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